The contribution rules for a Roth 401(k) are exactly the same as for a traditional 401(k). You can contribute a maximum of $18,000 in 2016 if you are younger than 50 years old. If you are 50 or older, you can make an additional "catch-up" contribution of as much as $6,000, for a total of $24,000. Those contribution limits are scheduled to adjust annually in line with inflation. 
